It is well known for flight actuator to include two mechanical load paths, one primary and the other secondary, with the latter intended to take up the load when the primary path has failed. In a typical prior art device, as shown in FIG. 1, when operating on the primary load path the loads are transmitted through a hollow ball or roller screw. The hollow screw houses a safety rod, also called a failsafe bar or tie bar, which is connected to the screw with a small amount of play. During normal operation of the screw, when the primary load path is working correctly, the secondary load path formed by this tie bar carries no load since there is no contact due to the small amount of play. However, in the event of a failure of the screw in the primary load path then the tie bar performs its failsafe function and ensures continuity of the transmission of loads by the actuator.
With reference to FIG. 1 a typical known flight control actuator of the trimmable horizontal stabiliser actuator (THSA) type includes a primary load path with a hollow screw 2 connected at its upper end to the aircraft via a Cardan joint system 4 joining with first aircraft structural elements 5. The primary load path further includes a nut assembly (not shown) mounted on the screw 2, and the nut assembly is connected to the stabiliser of the aircraft, this connection being achieved for example by another Cardan joint system.
The secondary load path is provided by means of a tie bar 3 which is within the screw 2. The tie bar 3 is terminated at its upper end by a male portion, in this case taking the form of a spherical head 7, which is mounted within a recess on a fastening piece 8. The fastening piece 8 is connected to the structure of the aircraft via second aircraft structural elements 9. The known system may also include some means for preventing motion of the nut assembly relative to the screw 2 and/or for fixing the stabiliser in place when the primary load path fails. Thus, the lower attachment, of which the nut assembly is a part, could also include secondary load path elements used when the primary load path fails.
A flight actuator with the basic features discussed above can be found in the prior art, for example in U.S. Pat. No. 8,702,034 and in US 2013/105623.
US 2013/105623 discloses a device for detecting the breakage of a primary path in a flight control actuator, said actuator having a primary path comprising a rotary hollow screw and a secondary path comprising a safety rod that reacts the load passing through the screw. The device of US 2013/105623 comprises a position sensor, connected to the screw, to measure information representative of the angular position thereof, and a disconnection system able to disconnect the screw position sensor in the event of relative movement of the rod with respect to the screw if there is a break in the primary path. Thus, when the primary path fails the disconnection system disconnects the screw position sensor and it is possible for the pilot to be alerted of a primary path failure. Advantageously this prior art system does not need the addition of new sensors to detect the primary path failure, since the position sensor is generally already present for the purpose of determining the position of the screw to thereby determine the actuator position.
Viewed from a first aspect, the invention provides a check device for a flight actuator primary load path failure detection device of the type that disconnects a position sensor from the primary load path in the event of a primary load path failure, the check device comprising: a mechanical linkage for simulating disconnection of the position sensor by permitting relative movement of at least first and second mechanical parts of the actuator that are unable to move relative to one another in normal use without failure of the primary load path, wherein these first and second mechanical parts include a first mechanical part with movement detected by the position sensor of the primary load path failure detection device.
It is known in the prior art for various systems to be provided that give a visual indication or an electrical signal when the primary load path fails. In the example described above in US 2013/105623 a position sensor is disconnected from the primary load path in the event of a primary load path failure. It is of course important to be able to establish with a high degree of certainty that the primary load path failure detection device is operating correctly. If the primary load path were to fail without this becoming known, then there would be a potential for a catastrophic failure of the aircraft should the secondary load path also fail. By the use of a mechanical linkage as set out above it is possible to test both mechanical elements and also electrical elements of the primary load path failure detection device, since the system uses a mechanical movement of the relevant parts and this can be checked by means of electrical signals from the position sensor. No additional sensors are required since the check device makes used of the same sensor as the primary load path failure detection device.
The primary load path failure detection device may be of the type that includes the position sensor as a first position sensor for detecting the position of the primary load path based on movement of the first mechanical part, and also includes a second position sensor for detecting the position of the primary load path based on movement of the second mechanical part. Advantageously the two position sensors and the associated mechanical parts may be elements that are already present in the flight actuator system, hence avoiding the need to introduce additional mechanical or electrical parts for implementation of the primary load path failure detection device. This also minimises the additional parts required for the check device.
The primary load path failure detection device may be of the type that includes a releasable element normally coupled between the mechanical parts and acting to inhibit relative movement thereof and this releasable element may be used as a part of the check device. Thus, the check device may incorporate a releasable element of the primary load path failure detection device, this releasable element being coupled between the mechanical parts as noted above. The releasable element is advantageously arranged to be disconnected upon failure of the primary load path, wherein the disconnection of the releasable element disconnects the position sensor from the primary load path, optionally by disconnection of the first mechanical part from the primary load path.
The releasable element may be a releasable pin element, for example a pin that is in normal use coupled between elements of the primary load path and the secondary load path, and is released upon failure of the primary load path. One possible form for this releasable pin element is a breakable pin (or fuse pin) arranged to be broken when load is transferred from the primary load path to the secondary load path. A suitable breakable pin is disclosed, for example, in US 2013/105623. In this case the release of the pin takes the form of shearing of a part of the pin originally held by the secondary load path, and subsequent freedom of the pin to rotate.
The position sensor, and optionally the second position sensor if present, may be any suitable sensor type, for example a sensor for detecting an angular position of a mechanical part, or a sensor for detecting a linear motion, where the angular position or the linear motion results from a change in position of a drive element of the primary load path, which may for example be a screw.